﻿// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace Microsoft.AspNetCore.Razor.Language.Syntax.InternalSyntax;

internal class SyntaxListPool
{
    private ArrayElement<SyntaxListBuilder>[] _freeList = new ArrayElement<SyntaxListBuilder>[10];
    private int _freeIndex;

#if DEBUG
    private readonly List<SyntaxListBuilder> _allocated = new List<SyntaxListBuilder>();
#endif

    internal SyntaxListPool()
    {
    }

    internal SyntaxListBuilder Allocate()
    {
        SyntaxListBuilder item;
        if (_freeIndex > 0)
        {
            _freeIndex--;
            item = _freeList[_freeIndex].Value;
            _freeList[_freeIndex].Value = null;
        }
        else
        {
            item = new SyntaxListBuilder(10);
        }

#if DEBUG
        Debug.Assert(!_allocated.Contains(item));
        _allocated.Add(item);
#endif
        return item;
    }

    internal PooledResult<TNode> Allocate<TNode>() where TNode : GreenNode
    {
        var builder = new SyntaxListBuilder<TNode>(this.Allocate());
        return new PooledResult<TNode>(this, builder);
    }

    internal void Free(SyntaxListBuilder item)
    {
        item.Clear();
        if (_freeIndex >= _freeList.Length)
        {
            this.Grow();
        }
#if DEBUG
        Debug.Assert(_allocated.Contains(item));

        _allocated.Remove(item);
#endif
        _freeList[_freeIndex].Value = item;
        _freeIndex++;
    }

    private void Grow()
    {
        var tmp = new ArrayElement<SyntaxListBuilder>[_freeList.Length * 2];
        Array.Copy(_freeList, tmp, _freeList.Length);
        _freeList = tmp;
    }

    public SyntaxList<TNode> ToListAndFree<TNode>(SyntaxListBuilder<TNode> item)
        where TNode : GreenNode
    {
        var list = item.ToList();
        Free(item);
        return list;
    }

    public readonly struct PooledResult<TNode> : IDisposable where TNode : GreenNode
    {
        private readonly SyntaxListBuilder<TNode> _builder;
        private readonly SyntaxListPool _pool;

        public PooledResult(SyntaxListPool pool, in SyntaxListBuilder<TNode> builder)
        {
            _pool = pool;
            _builder = builder;
        }

        public SyntaxListBuilder<TNode> Builder => _builder;

        public void Dispose()
        {
            _pool.Free(_builder);
        }
    }
}
